1. Field of the Invention
The present invention relates to a method for manufacturing an indium tin oxide (ITO) target and methods for preparing indium oxide powder (In2O3) and tin oxide powder (SnO2). More particularly, the present invention relates to a method for manufacturing a high-density ITO target which is used in vacuum deposition of a high-quality transparent electrode layer of a display such as a liquid crystal display (LCD), electroluminescent (EL) display, and field emission display (FED), and methods for preparing indium oxide powder and tin oxide powder used in the manufacture of the high-density ITO target.
2. Description of the Related Art
Due to their conductivity and transparency with respect to visible light, ITO films with the composition of In2O3 and SnO2 in a ratio of about 9:1 have been widely used as a transparent electrode film for an LCD, EL, or FED. In general, such an ITO film is coated on an insulating substrate such as a glass substrate by sputtering an ITO target. The ITO target is manufactured by molding ITO powder into a predetermined shape, for example, a rectangular parallelepiped shape, followed by sintering at a high temperature. To form a high-quality ITO film on the substrate by sputtering, the ITO target needs to have a high sintering density. If a low-density ITO target is used to form an ITO film by sputtering, nodules are easily generated on the target surface, thereby lowering the quality and yield of the resulting ITO film.
For this reason, a high-density ITO target is required to form a high-quality, transparent ITO electrode. To form such a high-density ITO target, ITO particles should be of an appropriate primary particle diameter. In general, the ITO particle diameter is inversely proportional to the target's sintering density. Therefore, the particle diameter should be reduced to increase the sintering density of the target. A method that is currently available for forming a high-density target having an approximately theoretical density is to reduce the particle diameter to a nano-scale. To manufacture a high-density target, it is important to adjust the ITO particle diameter to be uniform, as well as to reduce the particle diameter, for the following reasons. If a primary particle diameter of the ITO particles is too small, it is difficult to grind the particles after hydroxide calcination even though the driving force for sintering increases sufficiently for higher sintering density due to an increased specific surface area. It is also difficult to obtain a large molded body due to stress caused from the generation of many fine pores between the particles during target molding. In contrast, if a primary particle diameter of the ITO particles is too large, the fluidity and molding properties of the powder are improved, whereas the driving force for particle sintering is too low, so that pores between the particles become greatly enlarged, thereby increasing the energy requirement for removing the pores. For these reasons, to manufacture a high-density ITO target, the particle diameter should be fine and within a narrow range, and it should be easy to grind secondary particles.
A vapor phase method known for fine powder synthesis has been attracting attention as a method for nano-sized powder synthesis, but is limited to small-scale production of specific powder due to the difficulty of large-scale production. In this method, after powder synthesis, the particle diameter is reduced by grinding. In other words, the particle diameter of secondary particles rather than primary particles, which agglomerate to form the secondary particles, is controlled.
A liquid phase method has been used as a general method of large-scale powder production. Among other liquid phase methods, a precipitation method has been especially widely used to prepare ITO powder by precipitating metallic ions in a solution using a precipitant. In the precipitation method, the powder's characteristics are dependent upon the solution concentration, the reaction pH, the reaction temperature, the type of precipitant, the rate of adding a precipitant, etc.
The inventors of the present invention have discovered that the concentrations of the indium solution and tin solution are an important factor affecting the characteristics of the In2O3 powder and SnO2 powder prepared by precipitation. However, none of the methods of the prior art have limited the concentration of the indium solution and tin solution for precipitation. As a result, until now, it has been highly difficult to control the surface area and average particle diameter of the In2O3 powder and SnO2 powder even when controlling the pHs of the indium solution and tin solution, the temperature of precipitation reaction, the type of precipitant, the rate of adding the precipitant, etc. Accordingly, until now, it has been difficult to manufacture a high-density ITO target using the In2O3 powder and SnO2 powder prepared by those methods.